Is Global Warming Cooler than Expected?

LONDON – Several leading authorities on climate change have given a guarded welcome to research suggesting the Earth may warm more slowly than scientists had expected.

An international research team led by Alexander Otto of the Environmental Change Institute at the University of Oxford has reported its conclusions in the journal Nature Geoscience.

The Earth is now warming faster than at any time in the last 11,000 years, but scientists do not understand clearly why the atmosphere has warmed less than they expected over the last decade or so – and more slowly than in the 1990s.

The researchers looked at how the last decade would affect the long-term sensitivity of the climate to a doubling of atmospheric concentrations of carbon dioxide and suggest the Earth will warm more slowly than expected this century.

But even though the heating may be slower, giving politicians more time to act, the scientists still believe temperatures will eventually climb to 4°C above pre-industrial levels, well above danger levels.

Clive Hamilton is professor of public ethics at Charles Sturt University in Australia, and author of Requiem for a Species: Why we resist the truth about climate change. Published in 2010, it says climate change will bring about large-scale, harmful consequences for life on Earth which it is too late to prevent.

"The study should certainly be taken very seriously, although we will need to see over the next year or two how well it stands up to scrutiny," Hamilton said. "Let's hope they are right; it's the first bit of good news we have had a for a long time."

Still guesswork
Geoff Jenkins is the former head of climate change prediction at the UK Met Office's Hadley Centre for Climate Prediction and Research. He said: "Until we actually understand why the global temperature rise has paused over the last decade – and we don't yet – it's still guesswork what the implications are for climate sensitivity and hence the future projections.

If the pause is a "correction" to a naturally-boosted rise over previous decades, then the climate's sensitivity to carbon emissions may indeed be lower than the Intergovernmental Panel on Climate Change's central estimate, suggesting that future rises will be towards the lower end of the range, he added.

But if the pause is a temporary natural offset to the man-made rise, then this offset would disappear at some stage and put the globe back on the central estimate track, he said. "I don't see how we can say which it is until we understand the reason for it."

Many governments have promised to try to limit atmospheric warming to no more than 2°C above pre-industrial levels, believing that threshold will prevent dangerous climate change.

Image Courtesy of NOAA

Previous estimates showed that would require global greenhouse gas emissions to peak by 2020 and then fall, a prospect that seemed out of reach while emissions continue to rise fast.

Another member of the research team, Myles Allen of ECI Oxford, said the findings offer some optimism. "Prior to this, a lot of us were feeling quite gloomy that whatever we did, we'll go over 2°C", he said. "It's not a foregone conclusion any more."

That means the protracted UN climate negotiations could still produce a workable agreement. If a deal enters force in 2020 and leads to rapid emissions cuts, "there remains a good chance we could hit the 2°C target," Allen said.

Into the oceans
Explanations for the current pause are the masking effects on greenhouse gases of pollution, the seasonal weather-disrupting El Niño phenomenon, and slight variations in the Sun's output. It also appears that much of the heat has been going into the oceans, rather than into the atmosphere.

Otto and his colleagues estimated how much warming, with rising emissions, the world is likely to experience by the time carbon dioxide levels have doubled since pre-industrial times. Their most likely estimate is 1.3°C, not the rise of up to 1.6 °C suggested by previous studies.

"In the short term, there is maybe a bit less warming than expected," Otto said.

On such a contentious subject some scientists would argue for caution in evaluating the research. A rise of 4°C in global average temperatures could mean far higher regional rises. The oceans are likely one day to give up much of the heat they are now absorbing, which will return to the atmosphere.

So warming could accelerate fast and unpredictably. And on present trends, fossil fuel use is likely to go on increasing for several decades at least.

There are qualifications and conditions in the research, "ifs" and "buts" which suggest there may be unknown unknowns ahead. And its central conclusion is that the world is still warming – slower, perhaps, than expected, but still much too fast for comfort.

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.